Level attraction in a microwave optomechanical circuit

TL;DR

This paper demonstrates the phenomenon of level attraction in a microwave optomechanical circuit, revealing how negative energy modes lead to mode convergence and exceptional points, with potential for topological applications.

Contribution

The authors develop a theoretical framework for level attraction and experimentally realize it in a superconducting microwave optomechanical system with engineered dissipation.

Findings

Observation of level attraction between mechanical and microwave modes

Identification of two exceptional points with topological significance

Experimental control of dissipation to induce mode convergence

Abstract

Level repulsion - the opening of a gap between two degenerate modes due to coupling - is ubiquitous anywhere from solid state theory to quantum chemistry. In contrast, if one mode has negative energy, the mode frequencies attract instead. They converge and develop imaginary components, leading to an instability; an exceptional point marks the transition. This, however, only occurs if the dissipation rates of the two modes are comparable. Here we expose a theoretical framework for the general phenomenon and realize it experimentally through engineered dissipation in a multimode superconducting microwave optomechanical circuit. Level attraction is observed for a mechanical oscillator and a superconducting microwave cavity, while an auxiliary cavity is used for sideband cooling. Two exceptional points are demonstrated that could be exploited for their topological properties.